The present invention relates to a method of improving high temperature stability of platinum silicide (hereinafter referred as PtSi) film on a silicon substrate (hereinafter referred as a PtSi/Si structure), and in particular relates to a method of improving high temperature stability of a PtSi/Si structure by the implantation of fluorine-containing ions.
Platinum silicide (PtSi) has been widely used in silicon based devices and integrated circuits. It forms stable and reproducible Schottky contact on lightly doped n-type silicon and ohmic contact on p-type silicon at room temperature and heavily doped n-type silicon. The PtSi/p-Si structure can also be used as an infrared detector at a low temperature (77.degree. K.). Since PtSi has a resistivity of 30.mu..OMEGA.-cm, it has been suggested for use as a gate and interconnection metal over polysilicon. However, the high temperature stability of PtSi has always been questionable.
The often quoted high temperature instability of PtSi on silicon apparently arises from the fact that the lowest eutectic point in the Pt-Si system occurs at 830.degree. C. Even after the complete consumption of metal to form silicide at a low temperature, an agglomeration and resistance increase at a temperature of 700.degree. C. or higher has been reported. The high temperature instability of the PtSi has also been observed in the degradation of the PtSi/n-Si Schottky diode. Annealing such diodes to a temperature of 700.degree. C. or higher will cause large reverse leakage current and non-ideal forward current-voltage characteristics with lowered Schottky barrier height and an ideality factor much larger than unity. This is due to the reason that the Pt from the PtSi has diffused into the silicon substrate.
Up to the present, the only method that can slightly improve the high temperature instability associated with the PtSi/Si structure is to produce a film with a Si/Pt atomic ratio higher than that in the eutectic to bypass the eutectic composition and hence to improve the stability. This method is achieved by co-sputtering silicon rich alloy. The co-sputtered silicon rich alloy prevents the agglomeration of the platinum silicide, but increases the resistivity and decreases the Schottky barrier height of the film. Electrical properties, such as forward and reverse current-voltage characteristics, of the PtSi/Si structure still degrade after high temperature annealing. Furthermore, this method is not compatible with the modern VLSI process.